How can I stretch portion of a model along a plane/axis?
First you have to configure Blender to work with measurements, enable snap, select increment option (if I am not mistaken enable snap is not needed but do it just in case) then select the part that you want and press G, Y, - (if you want to move it to the right, don't add a - symbol, + means right, but you don't need to place + because it's implicit - blender assumes that) then the value that you want to add (1 or 2 for example). For a more thorough explanation, see this tutorial: https://www.youtube.com/watch?v=kOxN4CHeBPQ